1. Field of the Invention
The present invention relates to an image processing technique in an image capture apparatus such as a digital camera or digital video camera and, more particularly, to a white balance control technique in water.
2. Description of the Related Art
A signal output from an image sensor is A/D-converted into a digital signal, and the image is divided into a plurality of blocks. The block signal is formed from chrominance signals of R (Red), G (Green), and B (Blue). The color evaluation values of the block are calculated using, for example, equations (1):Cx[i]=(R[i]−B[i])/Y[i]×1024Cy[i]=(R[i]+B[i]−2G[i])/Y[i]×1024   (1)
(where Y[i]=R[i]+2G[i]+B[i])
When the color evaluation values Cx[i] and Cy[i] fall within a preset white detection region, it is determined that the block is white. The integral values sumR, sumG, and sumB of color pixels falling within the white detection region are calculated to calculate white balance coefficients in accordance with equations (2):WBCo_R=sumY×1024/sumR WBCo_G=sumY×1024/sumG WBCo_B=sumY×1024/sumB  (2)
where sumY=(sumR+2×sumG+sumB)/4
When capturing an image in water, the long-wavelength range of light readily attenuates under the influence of the spectral transmittance of water. The ratio of the R light component to the G and B light components decreases, and the image becomes bluish. In water, as the distance from the water surface (depth from the water surface) to the camera (object) becomes longer, the R color component of natural light entering water from the outside decreases. It therefore becomes difficult to hold the tracking performance of the white balance.
Japanese Patent No. 4144390 discloses a technique of setting an adjustment coefficient used in white balance adjustment before image capturing, and controlling a different white balance adjustment coefficient based on depth information detected by a depth indicator for detecting the depth where the camera is positioned.
However, in the conventional technique disclosed in Japanese Patent No. 4144390, the user needs to capture an object of an achromatic color such as white or gray prepared in advance, which is troublesome and cumbersome. Also, the camera needs to be equipped with, for example, a depth indicator for detecting the depth of water, and the camera cost rises.